The present invention is concerned with a MgO impregnated activated carbon and its use in an improved process for making a refined edible vegetable oil. The present invention is also concerned with a method of decolorizing a crude vegetable oil or a degummed vegetable oil, and removing organic acids therefrom. The present invention also deals with a method of preparing the MgO impregnated activated carbon. More particularly, the improved process of the present invention for making a refined edible vegetable oil comprises the steps of degumming a crude vegetable oil, passing the degummed vegetable oil through a bed of granular activated carbon impregnated with from about 1.0 percent to about 15.0 percent by weight of MgO, and subjecting the treated oil to steam distillation at reduced pressure.
The vegetable oils to which the present invention is applicable are such edible vegetable oils as soya or soybean oil, corn oil, cottonseed oil, peanut oil, sesame seed oil, rapeseed oil, olive oil, palm oil, palm kernel oil, coconut oil, and babassu oil, among others.
The improved refining process and treatment methods of the present invention have proven especially suitable in refining of soybean oil, and they are, therefore, particularly applicable thereto. Soybean oil is, moreover, the most important vegetable oil produced in the United States, comprising about 82% of the present total annual vegetable oil production. Thus, production of soybean oil in the United States is an important and extensive industry, with current annual production of edible soya oil being approximately 9.5 billion pounds. While crude soybean oil is stable and nonreverting in nature, it has a dark color and a strong odor and taste which make it regarded as unpalatable in that state. Consequently, a number of techniques have been employed previously in the art to refine the crude soybean oil. The resulting product, while initially a light colored oil with a bland and agreeable flavor, in many cases unfortunately reverts by stages to more unpleasantly flavored forms after standing for a short period of time.
In accordance with the improved refining process of the present invention, it is possible to eliminate two conventional refining steps which are cumbersome and wasteful. Despite the eliminated steps, the end product refined oil is acceptable with respect to prevailing industry standards for taste, odor, and color, and is, moreover, storage stable over the normal shelf life period of from one to three or more months. The end product refined oil produced in accordance with the present invention is thus comparable to oils produced by conventional refining processes in these respects.
The MgO impregnated activated carbon of the present invention is able to remove not only substantial quantities of free fatty acids from a crude vegetable oil, but is also able to remove substantial quantities of phospholipids and peroxide compounds from the crude vegetable oil.
Free fatty acids, referred to herein generally as organic acids, are present in crude vegetable oils and may result, as well, from hydrolysis of the crude vegetable oil responsive to a number of conditions. The free fatty acids may be saturated, for example, caproic, lauric, palmitic, stearic, and myristic acids, or unsaturated, for example, oleic, linoleic, and linolenic acids.
Phospholipids, or phosphatides, are lipoid substances that occur in cellular structures and contain esters of phosphoric acid. The aminophosphatides, or lecithins, which are mixed esters of glycerol and choline with fatty acids and phosphoric acid, are especially common. For example, the phospholipid content of crude soybean oil ranges from 1.1 to 3.2% by weight, and averages 1.8%.
Free fatty acids are conventionally removed by means of caustic refining, as well as steam distillation under reduced pressure, both of which are described in detail below. Phospholipids are conventionally removed by means of degumming, which is also described in detail below.
Use of the MgO impregnated activated carbon of the present invention provides not only an improved process for making a refined edible vegetable oil, but also may provide a method of decolorizing a crude vegetable oil or a degummed vegetable oil, as well as of removing organic acids from such oils. The present invention thus provides a complete refining process for producing edible vegetable oils, as well as more intermediate processes for improving the quality of a crude vegetable oil or degummed vegetable oil with respect to decolorizing thereof and removing organic acids therefrom.
Conventional processes heretofore employed for refining vegetable oils, particularly soybean oil, have employed a number of distinct treatment steps. However, most often these have consisted of degumming, alkali neutralization, water washing, bleaching, and deodorizing, employed in that order. See, for example, U.S. Pat. No. 3,629,307.
1. The step of degumming removes various mucilaginous products, primarily protein or albuminoid substances and phospholipids, from the crude vegetable oil. These phospholipids, primarily lecithin, cephalin and inositol phosphatide, are primarily responsible for the rather strong and bitter flavor and aroma of the crude oil. They are responsible, moreover, for fouling of processing equipment employed in subsequent refining operations, if they are not successfully removed. The degumming process is primarily carried out at the extraction mill, where alkali refining may or may not be carried out. To a much lesser extent, degumming may be done by the refiner at another location. In general, the degumming process consists of adequately mixing with the crude vegetable oil, an organic acid such as phosphoric acid or acetic acid, followed by a little water. The resulting hydrated, mucilaginous globules are subsequently removed from the oil by centrifuging. The step of degumming may also be carried out without the use of acid, by simply adding water. Both types of degumming will be described in more detail hereinafter. While substantially all of the phospholipids should be removed, to a level at least below 2.0 p.p.m., as phosphorus, it has not been considered possible to accomplish such a result by conventional water degumming alone. Unless substantially all of the phospholipids are thus removed, a dark colored oil will be produced by decomposition of the remaining phospholipids at the elevated temperatures encountered during the final step of vacuum steam stripping and deodorizing. This dark colored material is very difficult to remove by ordinary refining or bleaching and imparts an off-flavor to the refined vegetable oil final product. In addition, the phospholipids tend to chelate any metal ions contained in the vegetable oil being refined, and will tend to carry these over into the refined oil final product, where they can cause undesired oxidation of the refined vegetable oil final product. Moreover, the phospholipids recovered, particularly lecithin, continue to possess good market value as a by-product for sale in nonrelated fields, for use, for example, as an emulsifying agent.
Various methods of degumming have been employed in the past, including the use of various aliphatic and aromatic hydrocarbon and other compositions which are solvents for the vegetable oil, but nonsolvents for the phospholipids and other mucilaginous products. Acetone is an example of a suitable solvent. As the solvent is added to the vegetable oil, the decreased solubility of the phospholipids and other impurities causes them to precipitate out of the oil. Separation can then be acheved simply by filtration. The separated oil is then treated to remove the added solvent, for example, by distilling under a moderate vacuum. See, for example, U.S. Pat. No. 2,117,776. However, such methods have the serious drawback of requiring the use of often hazardous solvents.
The preferred method of degumming for use in the improved refining process of the present invention is one whereby the phospholipids and other mucilaginous products are simply hydrated, precipitated, and separated, desirably by a continuous process. As before stated, an acid such as 85% phosphoric acid is also used before the addition of the water. The amounts used may vary from 300 to 2,000 p.p.m. by volume of oil. The amount of water may be from 1.0 to 3.0% by volume. The temperature may be from 100.degree. to 160.degree. F. The refining process of the present invention may employ either (1) the simple degumming method using water alone, (2) the acid degumming method using water and acid together, or (3) a combination or sequence of the degumming methods (1) and (2). The mixture is introduced into a continuous centrifuge in which it is heated and caused to circulate continuously, whereby the mucilaginous products are completely hydrated and the aqueous phase containing these hydrated mucilaginous products is finally discharged. See U.S. Pat. No. 3,206,487.
2. The second step in the conventional oil refining process is alkali neutralization of the oil to remove free fatty acids and other impurities. Usually, this neutralization is accomplished simply by treating the oil with an aqueous solution of sodium hydroxide or other strongly alkaline reagent. The free fatty acids in the oil, generally present in amounts of from 0.5 to 3.0% by weight, are removed as precipitated soaps produced by the reaction of the fatty acids and alkaline reagent. The soap thus formed may be removed by centrifuging and the separated soapstock disposed of in some manner. However, handling of these soapstocks has presented considerable problems to the vegetable oil refiner. Usually, these soapstocks are acidulated and free fatty acids are recovered. Nevertheless, waste-products are produced which cannot be reaily disposed of without creating problems of environmental pollution. As a final step, the oil is then washed with water to remove virtually all traces of soap, and the oil is then dryed to remove any dissolved or emulsified water which may be present.
3. The third step in the conventional vegetable oil refining process is bleaching to remove pigments remaining in the oil after the previous refining steps. Such pigments comprise the carotenoids and chlorophyll, among others. Typically, the bleaching step is carried out under vacuum at a moderate temperature in the range of 210.degree. to 250.degree. F., and in the presence of an activated earth such as fuller's earth, perhaps admixed with a lesser amount of activated carbon. After the bleaching has been carried out, it is necessary to filter out the bleaching earth and activated carbon and pigment products adsorbed thereon. It has been found that during bleaching some free fatty acid products are generated and that the acid value of the oil is raised to about double that existing at the end of the alkali neutralization process.
4. The fourth step in the conventional refining of vegetable oils is deodorizing. During this step live steam is passed through the vegetable oil while it is maintained under a high vacuum and at elevated temperatures. The temperature usually ranges from 460.degree. to 530.degree. F. and the vacuum is maintained at 4 to 6 mm Hg. The process may require from one and onehalf to seven hours. During the process most of the free fatty acids remaining in the vegetable oil are distilled off. Most of the remaining pigment products are destroyed during this step as well. The acid value and color of the oil are thus improved, and the odor and flavor are made acceptable. However, if any appreciable quantity of phospholipids remains, the elevated temperatures experienced during this step would result in a darkening of the oil. For most vegetable oils it has been considered necessary to utilize both alkali neutralization as well as deodorization in order to remove most of the free fatty acid content of the vegetable oil, as well as to get a bland-tasting and odor free edible oil.
The conventional deodorizizing step has been improved by variation of the parameters involved and other modifications. See, for example, U.S. Pat. No. 3,506,696.
The vegetable oil refining method of the present invention is an improvement over the conventional refining process described in the paragraphs above since it eliminates the conventional alkali neutralization and water washing step, and the conventional bleaching step. This conventional process step elimination is possible because the impregnated activated carbon treatment of the present invention reduces the phospholipid and free fatty acid concentration of the degummed oil sufficiently to result in a final product which is both stable and acceptable from the standpoint of taste, color and odor. In addition, the final product is satisfactory as a consequence of the removal of other impurities in the oil, especially peroxide compounds, by the impregnated activated carbon treatment step.
In addition to the conventional refining process described above, other, often more direct, methods have been put forward in the art as improved methods of vegetable oil refining. For example, U.S. Pat. No. 2,746,867 describes a two step refining process comprising a first step of carefully controlled partial degumming by means of hydration, followed by steam deodorizing at a moderate temperature. However, this process is intended to leave at least some of the free fatty acids in the product. Similarly, U.S. Pat. No. 2,117,776 describes a two step process comprising removal of the phospholipids from the crude oil, preferably by precipitation with a non-solvent, followed by high vacuum-short path distillation of the oil.
As already noted above, it is known to employ activated carbon conventionally as a bleaching agent, that is, as a decolorizing agent to remove various pigment products. When employed as a bleaching agent, the activated carbon is typically utilized in powder form in a batch or continuous batch-type operation. Conventionally, such use takes place before the vacuum distillation deodorization step. Nevertheless, the art has preferred to employ activated clays as bleaching agents rather than activated carbons due to their greater cost effectiveness, a result of the much greater holding capacity of the activated carbons for the vegetable oil, as compared to the activated clays.
However, it is known to employ activated carbons in various ways in vegetable oil refining processes. For example, John P. Harris and Bernard N. Glick, in "Crude Cotton Oil Filtration", Oil & Fat Industries, pp. 263-265, September, 1928, suggest activated carbon filtration of crude cotton oil to remove certain colloids and other impurities prior to the conventional refining process. U.S. Pat. No. 3,455,975, concerned with a refining process wherein deacidification and deodorization of glyceride oils is accomplished by distillation in a steam current under vacuum, also discloses decolorization pretreatment with artificially activated montmorillonite earth and activated carbon.
Finely divided activated carbon impregnated with, or admixed with, an alkaline material has been employed in purification of oils. See U.S. Pat. Nos. 1,105,744, 1,705,824, 1,705,825, and 2,105,478.
The MgO impregnated activated carbon of the present invention represents a novel catalyst composition. U.S. Pat. No. 3,817,874 discloses a method of forming high surface carbons by treating a porous carbon with MgO, among other materials, followed by heating of the carbon in the presence of CO.sub.2, and then by washing out of all the inorganic materials, but does not teach the impregnated activated carbon of the present invention.
A process for refining edible glyceride oils by treating them with activated magnesium oxide is disclosed in U.S. Pat. No. 2,454,937. Ordinary magnesium oxide, however, is stated to be inactive, and it is said that activation may be accomplished by heating the magnesium oxide with water to about 100.degree. C. for about one hour, filtering, drying and heating in the range of 350.degree. to 500.degree. C. for three hours or longer.
In contrast to methods heretofore employed in the art, the method of the present invention uniquely provides for a straightforward and efficient means of preparing refined vegetable oils having a reduced content of free fatty acids, phospholipids, peroxides and other impurities which would result in an unstable product and one unacceptable in color, taste and odor.